1. Field of the Invention
This invention pertains to welding of very hard to satisfactorily weld highly oriented thermoplastic materials and cast Nylon. The bead welding of this material requires very critical parameters of time, temperature and extension from cooled clamp jaws.
2. Description of the Prior Art
There are presently many patents and inventions relating to "bead" heat welding of plastic film with congruent edges. Most of these patents pertain to, plastic film containing or coated with copolymers, various plasticizers and other additives which are substantially distinct from polyester film containing no additives. Polyester is the generic name for polyethylene terephthalate. Mylar type D (DuPont TM) having high clarity, balanced biaxially orientated, heat set film is such a film.
The unique properties of this Mylar type D are a tensile strength of at least 25,000 p.s.i.; semi-rigidity; inertness; extremely long shelf life; contains no plasticizers; has a very high melt temperature, and is resistant to most chemicals, acids, alkalines, oils and other organics. It is relatively impermeable to moisture and oxygen and has a heat conduction and is inert from fungus attacks. Has extremely high initial tear strength and has unique dielectric properties. Mylar is used to store valuable documents and collector's items, film negatives and many other items affected by volatile plasticizers and additive compounds of the many other types of plastic films.
It is commonly realized, in the present state of the art, that sealing this type of polyester film with ultrasonic welding can lead to one or more of the following problems: brittle seals, opaque recrystallization of the sealed bead (a milky white appearance); loss of strength of the seal; heat warping of the unsealed surfaces, and decomposition of the plastic on portions of the seal. The melt temperature is very close to the decomposition temperature, thus portions of the bead seal may become overheated and decompose.
Applying the known art such as shown in U.S. Pat. No. 2,679,469 to Bedfore as issued on May 25, 1954; U.S. Pat. No. 3,321,353 to Zelnick as issued May 23, 1967; U.S. Pat. No. 3,330,716 to Zelnick as issued July 11, 1967; U.S. Pat. No. 3,326,735 to Beason, Jr. as issued June 20, 1967; U.S. Pat. No. 3,355,337 to Zelnick as issued Nov. 28, 1967; U.S. Pat. No. 3,736,219 to McTaggart as issued May 29, 1973; and U.S. Pat. No. 4,197,149 to Freitag as issued Apr. 8, 1980 to the seal of Mylar film without using the distance, time and temperature control mentioned or suggested in the following description causes unsatisfactory results as listed above. Any or all of the above results are adverse to the intended use of Mylar film. It is a purpose of this invention to provide a seal to effectively eliminate all of the above adverse results.